Undersea optical communication systems include land-based terminals containing transmitters and receivers connected by a cabled-fiber-transmission medium that includes periodically spaced repeaters, which contain optical amplifiers whose purpose is to compensate for the optical attenuation in the cabled fiber. In a bidirectional transmission system each repeater will generally contain two or more optical amplifiers, one for each of the oppositely-directed transmission paths. As the repeaters are usually placed undersea and away from power sources, power must be supplied remotely to the repeaters. The cabled fiber therefore usually contains a copper conductor to carry electrical power to the repeaters from the terminals. These undersea systems serve to carry optical communication signals (i.e., traffic) between the terminals. The traffic on these systems can consist of voice, data, television, Internet traffic, international telephone traffic, etc. Consequently, the revenue lost when the system is down can be significant. Therefore, these systems must have high reliability and availability.
Repeaters are typically housed in a pressure vessel that must withstand high undersea hydrostatic pressures and remain hermetic for at least 25 years. The pressure vessel must also be corrosion resistant or at least capable of being coated with an anticorrosion component. Suitable materials that are often employed include a high-strength grade of copper-beryllium and steel. The pressure vessel is generally quite large, often over 1000 cm in length, with a pressure vessel designed to house sixteen optical amplifiers for amplifying eight fiber pairs often being around 1500 cm in length.
To provide a hermetic seal and to prevent corrosion, particularly if steel is employed, fusion-bonded epoxy and polypropylene and polyurethane paint are coated directly onto the pressure vessel.